Synthesis, Characterization, Biodegradability And Toxicity Of [Cholinium][Amino Acid] Ionic Liquids

Ionic liquids (ILs) have been regarded as “green solvents”, due to their excellentproperties such as non-volatility and nonflammability. They showed an application potentialin various fields such as electrochemistry, separation engineering and organic synthesis.However, the traditional ILs such as imidazolium ILs were demonstrated to not be green. Forexample, a large amount of volatile organic solvents were used in the synthesis andpurification of ILs; they were toxic and poorly biodegradable. As a result, a group of[cholinium][amino acid]([Ch][AA]) ILs were synthesized by simple acid-base neutralization,starting from readily available and renewable choline hydroxide and amino acids. And thephysicochemical properties, biodegradability and toxicity of these ILs were characterizedsystematically.Eighteen [Ch][AA] ILs were prepared with more than95%yields, and their structureswere confirmed by FT-IR,1H NMR and element analysis. The water contents were less than0.08wt%in these ILs. No melting points were observed for all the ILs; the glass transitiontemperatures of these ILs ranged from-74to-10°C, and their decomposition temperatureswere in the range of150-203°C. These ILs were miscible with water, methanol, ethanol,i-propanol and benzyl alcohol, while being immiscible with acetonitrile, acetone, ethyl acetate,n-hexane, dichloromethane, chloroform and diethyl ether. At25°C, these ILs had theviscosity of121-5640mPa·s.As artificial compounds, the microbial breakdown of ILs is one of the most importantdegradation routes as well as the key step of the chemical elements circulation in biosphere.Therefore, the biodegradability of ILs works as an important evaluating indicator for theirenvironmental persistent and impact. The biodegradability of these ILs was evaluated by theclosed bottle (OECD301D) and CO2headspace tests (ISO:14593:2005). All these ILs couldbe classified as “readily biodegradable” because of the high levels of mineralization (62-87%)after28days.The half maximal inhibitory concentrations (IC50) of these ILs to acetylcholinesteraseand catalase were≥2451μmol/L and≥1.95mol/L, respectively. Both minimal inhibitoryconcentrations (MIC) and minimum biocidal concentrations (MBC) of the ILs towardEscherichia coli ATCC8739, Staphylococcus aureus ATCC29213, Salmonella enteritidisATCC14128and Listeria monocytogenes ATCC19115were≥23.4mmol/L, among whichMIC value of [cholinium][glutamic acid] to S. aureus ATCC29213was as high as750 mmol/L. Therefore, the toxicity of this class of ILs was pretty low.This study would not only lay the theoretical basis for applications of [Ch][AA] ILs, butalso provide guidance for rational design of novel green ILs.